unforum

[Agent Lex]

[Rogi Ocnorb]

Okay. Who's gonna do the Mythbusters submission?

I wanna see the crash-test-toddler get blown across the tarmac.

[MeCon]

now that i think of it, i believe that thebruce already answered the question. the helicopter would move with the carrier, or the earth, when in a sheltered location, but when its just hovering in the middle of the ocean the carrier would move from under it.

Sooo...that's it i guess.

[yanka]

[MeCon]

this stuff hurts my brain...and i love it.

[SirQuady]

[MeCon]

yes, i am sure that if carrier did move from under the copter, the helicopter would plunge into the sea because the air being pushed down by the rotors would not have anything to push against anymore

[thebruce]

I don't think it's so much that the pushed air under the copter is pushing against the carrier - what keeps the copter in the air is the energy required for the propellers to slice through the air above it and give it an upward thrust. Think of it this way - I don't think a helicopter hovering over the water would need to change its propeller speed if a carrier passed under it (nor would it begin to move with the carrier), and it would remain as is until the carrier passed. I could be wrong on that, but as far as my understanding of helicopter physics goes, the thrust is provided by the propellers pushing air down, not the pushed air providing resistance keeping up - that is to say, a hovreing helicopter wouldn't 'drop' if the carrier below it moved away. The air being thrust downwards andpushing against the deck would still have push against the air filling the space the deck was.

Or a better illustration, if a helicopter flies low over a changing landscape, it doesn't have to keep adjusting it's rotary speed based on the distance to the land below - that doesn't have an effect on its upward thrust.

So 1) the helicopter wouldn't "follow" the carrier just because the air it displaces is pushing against the deck, and 2) the copter wouldn't sink if the deck moves out from underneath it.
I believe that's true... but heck I coud be wrong

[Rogi Ocnorb]

I am a Mythbusters fan. Haven't seen that episode, yet. But I'm of the opinion that the plane would self-destruct before taking off (Or at least before the wind from the treadmill created enough lift to make it takeoff).

On the helicopter one... There is no downward force keeping it in the air. It's the same as any airfoil. As it moves through the air, pressure above the airfoil is reduced, providing lift from the higher pressure air under the airfoil. The faster it's moving, the greater the lift..

There is a certain lower bound on elevation that, if it's under, will introduce a ground effect as the wind from the blades and the venturi effect fight for equilibrium and the momentum of the air spreading out across the ground lowers the air pressure under the helicopter. You can see this effect with newer pilots coming in for a landing. They descend at a steady rate slow down rapidly as the pressure under the bird increases and then drop more rapidly, just before touchdown, because of the venturi effect, when the air gets moving well.

[Mikeyj]

The way the helicopter generates lift is analogous to that of planes or anything else that flies, the prop blades are shaped and angled so that as they rotate the air moves faster over the blades than under them, creating a pressure differential that sucks the helicopter into the air. The presence of the air craft carrier is irrelevant to the helicopter (other than for landing purposes). The helicopter is effectively moving forward at the same speed as the moving carrier only because it was moving forward and that speed before take off and the momentum is conserved. If the carrier sped up the instant the helicopter had taken off it would be left behind.

Interestingly despite the fact the hovering copter is no longer affected by the aircraft carrier, the reverse is not true as the down draft from the blades would affect fuel consumption - there's a nice illustration of this problem in the New Scientist's Last Word section from a few months ago. I particularly like the idea of training pigeons to synchronise free-fall.

[MeCon]

yes, it seems to me now that if the helicopter was in a sheltered area of air it would move with the carrier, but in the situation that the original question is asked in, there is no such shelter and therefore would not move with the carrier.

say, if the helicopter was hovering (for some reason) below decks in the protected pocket of air that the ship itself provides, it would move with the carrier.

[thebruce]

yeah, and my point of using a wall as a shelter on the deck was just to emphasize the vision that the helicopter is totally unconnected to the carrier - that is, take the helicopter hovering over the deck, then put a big wall in front of the helicopter on the deck. Without the wall's shelter, the copter would have to tilt forward to move with the carrier, but behind the wall in a pocket of air that (for arguments' sake) has no turbulence or air movement due to the wall, the helicopter would remain stationary to the carrier.

To use rowan's point, if the pilot were hovering over the deck freely, hi point of reference would be the water (as that's also effectively the carrier's point of reference for motion). If he flies beind a shelter, his point of reference would then shift to the carrier.
meh that's a little harder to picture.


Back to the ball in the car - if you toss the ball at the windshield, it'll move in relation to you, the car, your hand, etc, even though it's no longer attached, because it's in the pocket of air formed by teh car and moving with it. Toss the ball out the window, and the ball's then affected by the outside air and its relative frame of reference becomes the ground, and it gets blown back.

Of course, physics doesn't work by 'frame of reference', but rather it's about momentum and friction and all... in the car, everything moves with the car - no air movement or friction, and everything inside that's connected is really being pulled/pushed by the car's energy exerted against the ground. Really when you toss a ball in the car, the ball's motion is basically the car's momentum (to the point of release), plus your throwing strength (after release), minus the relative friction of the air in the car. If you toss it out the window, it still has the car's momentum, plus your throwing speed, minus the air friction produced by the difference in the ball's speed to the ouside air's (which is much greater than that produced by the difference between the air in the car and the ball in the car).

Back to the helicopter, it has a forward velocity, fighting against the air it flies through - whether it's over water or the carrier. When it flies behind a shelter producing an air pocket that is effectively traveling 'with' the carrier, it no longer has air to push through to maintain its speed, so the forward momentum it already has (assumedly having matched speed with the carrier) is enough to have it stationary relative to the carrier when behind the shelter.

I kind of picture a car driving into the back of a big moving truck on the highway. It has to match speed with the truck to reach the ramp, but as soon as it drives up, if it doesn't slow down relative to the truck itself, it'll fly through the front of the truck at the speed it was driving on the highway.

...fly a copter over a moving carrier and try to land behind a shelter, and the pilot would have to adjust the copter's trajectory really quick relative to the carrier once 'in the pocket', as it were, or it would crash into the shelter =)

[Rogi Ocnorb]

[thebruce]

I'm refering to it used in an observatory sense... what we call frame of reference is really how we observe objects in relation to ourselves in this case; so saying an object moves based on its frame of reference is really insufficient - what defines the object's frame of reference? In this case it's the relative motion of the helicopter and the carrier - frame of reference would be useful to the pilot, for deciding on how to fly the copter second by second, but physically, there's a reason -why- the pilot's frame of reference would change between flying in the open air above the carrier and flying into a sheltered region still unattached from the carrier.

That's what I meant.. in so many words
Frame of Referencewould be an application of physical laws, not a physical law itself... "physics doesn't work by f.o.r" was probably just bad phrasing =P
heck, even this entry could be bad phrasing

[Rogi Ocnorb]

I's just pokin fun
It's not an easy subject to navigate.
And Newton probably wouldn't have had any problem with it.

Edit: Maybe Newton wouldn't have agreed, so much.
It's in the "non-inertial reference frame" bucket and includes all manner of variables and fictitious forces. Like the fact that we're doing between 603,000 and 740,000 miles per hour at any given moment.